I. Field of the Invention
This invention relates to the separation and removal of water from a water-containing mixture or azeotrope with an extractant, as well as for the separation and removal of water from said extractant. In particular, it relates to the separation and removal of water from an alcohol-water mixture with an extractant, and for the regeneration of said extractant by the removal of water therefrom.
II. Background and Problems
Distillation operations in the petroleum and chemical industries have as their objective the separation of individual components from volatile liquid mixtures consisting, for the most part, of several well defined components. The principles of distillation and rectification are well known. Distillation, the broader term applied to vaporization processes, is that type of process wherein the vapor evolved from a still is recovered by condensation. Rectification is a distillation carried out in such manner that the vapor evolved from the still is contacted with a condensed portion of vapor previously evolved from the same still (reflux). The resulting transfer of material and interchange of heat between the vapor and liquid produces greater enrichment of the vapor with the more volatile components than is possible by simple distillation utilizing the same amount of heat.
Subsequent to the separation by rectification of a more volatile component from a liquid admixture, the separated product component often is not in as pure a form as is desired. For example, after distillation or rectification of an alcohol from an aqueous admixture, the recovered alcohol generally contains more water than is desirable for many purposes. Alcohols such as ethanol, n-propanol, or isopropanol, thus form azeotropic or constant boiling mixtures with water for which reason there is a limitation on the amount of water which can be removed from the alcohol by ordinary distillation, or rectification. Thus, once an azeotrope of the alcohol and water is formed the boiling point of the remaining alcohol-water system does not change as vapor is generated and removed. Consequently, to obtain anhydrous alcohol it is necessary to use means other than ordinary distillation, or rectification to remove the last bit of water from the azeotrope, or alcohol-water system.
Present methods for the separation of water from a compound, e.g. alcohol, requires distillation, or rectification, of the aqueous alcohol admixture in a multistage still to separate and recover an alcohol-water azeotropic composition from the still. To the azeotropic composition is then added an agent such as an entrainer, adsorbent, or extractant to assist in removing the residual water from the alcohol-water azeotropic composition. Various schemes are known for removing the final amount of water from the alcohol-water azeotropic composition, to wit:
Using an entrainer: The water, alcohol, and entrainer, e.g. benzene, are distilled in a multistage distillation column. The alcohol passes downwardly through the column and is removed from the bottom of the column. The water and entrainer ascends within the column, and is removed from the top of the column as a vapor mixture, and then condensed. The water and entrainer are then separated in a decanter and the water is removed from the system. The entrainer is recycled to the column and reused.
Using an adsorbent: The water and alcohol are passed through a bed of the adsorbent where water is adsorbed on the surface or in the pores of the adsorbent. The water is usually removed from the adsorbent by contact with a hot gas. The adsorbent may be reused, or if fermentable it may be fermented to make additional alcohol.
Using an extractant: The water, alcohol and extractant are distilled in a multistage distillation column. The alcohol passes upwardly through the column and is removed from the top of the column. The water and extractant pass downwardly through the column and is removed from the bottom of the column. The water may be removed from the extractant by boiling the water out of the extractant-water mixture, and the extractant then reused.
The use of an extractive agent provides a relatively simple and economical process for removing water from these and other azeotropic mixtures. The removal of the last portion of water from the extractive agent itself however is also a most desirable objective. This however requires high temperature, or high vacuum, or both. It is thus necessary to remove the water from the extractant down to a very low concentration for the efficiency of the extractant in removing water from the alcohols requires the extractant to be dry, or water if present must be in very low concentration. High temperatures cause the extractant to deteriorate. High vacuum, on the other hand, requires expensive equipment and the expenditure of considerable energy.